Why power/weight and not torque/weight?
Discussion
It's dependant on the gearing - a car with the same power but lower torque will be able to rev higher. The answer is they should in theory be able to accelerate at the same rate. In practice the car with more torque will be more relaxing to drive and require less gear changes to achieve the same performance. Both cars would have the same top speed.
What matters is how much power the car puts on the road as it accelerates through the gears. And this depends on the gearing of the car as well as the size and shape of the engine's power curve. The figures you normally see quoted are the peak power figures produced at the top of the engine's rev range, but most engines only produce this much power very occasionally and briefly. The rest of the time the engine is working at lower revs. So in the real world the faster car will be the one that produces more power (relative to the weight of the car) within the rev range that is actually used.
If you compare two engines with equal power output, one which produces relatively little torque but loads of revs, and one which produces loads of torque at low revs but doesn't rev well, the torquey engine will generally have more power mid-range. This means that the torquey engine will typically be faster in the real world.
That doesn't mean the torque matters more than power, just that the torque figure gives you an indication of the shape of the power curve, which you don't get just from the peak power figures.
If you compare two engines with equal power output, one which produces relatively little torque but loads of revs, and one which produces loads of torque at low revs but doesn't rev well, the torquey engine will generally have more power mid-range. This means that the torquey engine will typically be faster in the real world.
That doesn't mean the torque matters more than power, just that the torque figure gives you an indication of the shape of the power curve, which you don't get just from the peak power figures.
OK..
I was hoping to finally get an intuitive understanding of this but I don't think you can. You can get an intuitive feeling for what torque is "ie twisting force" but when you start talking about power then you need to start including things like engine speed, and gearing, which makes it harder to grasp without thinking about it.
The only way it makes sense to me is:
Torque is the thing that accelerates the car. Power is the ability to stay in one particular gear for longer (ie the ability to apply that torque at higher revs). With high torque but low power, the torque would drop off rapidly as the revs rise, so you have to change gear (and therefore use worse gearing). With high power, adequate torque is still available at higher revs so you don't have to change up for longer, hence you are using the lowest gear ratio for longer..
I think that kind of makes sense to me..???
I was hoping to finally get an intuitive understanding of this but I don't think you can. You can get an intuitive feeling for what torque is "ie twisting force" but when you start talking about power then you need to start including things like engine speed, and gearing, which makes it harder to grasp without thinking about it.
The only way it makes sense to me is:
Torque is the thing that accelerates the car. Power is the ability to stay in one particular gear for longer (ie the ability to apply that torque at higher revs). With high torque but low power, the torque would drop off rapidly as the revs rise, so you have to change gear (and therefore use worse gearing). With high power, adequate torque is still available at higher revs so you don't have to change up for longer, hence you are using the lowest gear ratio for longer..
I think that kind of makes sense to me..???
gregwatson said:
OK. So Torque accelerates. And obviously weight is a factor. So how come we use power/weight ratio as an indication of a car's ability to accelerate, and not the torque/weight ratio??
Torque pulls weight.
Power makes it accelerate.
With a suitable lever you could produce 1000lbft of torque easily enough ( 200lb person on a 5ft long bar ). But as a human being, you wont have any power to make it go anywhere fast.
A motorbike engine might produce 150bhp, with crap torque. Its ok for a lightweight bike, and it will accelerate quickly, but ask it to pull a car, and it has no chance.
The 2 do go hand in hand though to make a car accelerate quickly.
Diesels also make a lot of torque, but usually no power. So they can pull, and tow, but generally they dont accelerate quickly.
Im no expert but some simple ways to look at this as follows
Power is the rate at which torque is applied
Hi Torque with low power (typicaly Diesels) = Lots of gear changes
Hi Power with low Torque (typically motorbikes etc) = Lots of gear changes
Hi Power with High Torque (Big V8 Monaro) = Excitement with a great soundtrack
This is a balance; more power and torque usually come with more weight and you need more power and torque to accelerate the increased weight to its ultimate top speed. "the law of diminishing returns"
High Torque low gearing = acceleration (limited by Mass)
High Power high gearing = Higher top speed (limited by Cd the coefficient of drag)
Different horses for Courses. The laws of physics dictate that you cant get something for nothing and everything is limited by something .
Power is the rate at which torque is applied
Hi Torque with low power (typicaly Diesels) = Lots of gear changes
Hi Power with low Torque (typically motorbikes etc) = Lots of gear changes
Hi Power with High Torque (Big V8 Monaro) = Excitement with a great soundtrack
This is a balance; more power and torque usually come with more weight and you need more power and torque to accelerate the increased weight to its ultimate top speed. "the law of diminishing returns"
High Torque low gearing = acceleration (limited by Mass)
High Power high gearing = Higher top speed (limited by Cd the coefficient of drag)
Different horses for Courses. The laws of physics dictate that you cant get something for nothing and everything is limited by something .
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